1. Field of the Invention
The present invention relates to a digital network system which comprises voice (telephone) communication trunk exchanges and a plurality of multiplex communication apparatuses and, more particularly, to multiplex communication apparatuses used in such a system to realize digital compression transmission of voice signals with a high quality.
2. Description of the Related Art
There is shown in FIG. 9 a conventional digital network system in which private branch exchanges (hereinafter, which will be referred to merely as PBX's) 1 to 4 each having a relay function are connected through trunk lines 10, 20, 21, 30, 31 and 40 to conventional multiplex communication apparatuses 100, 200, 300 and 400, respectively. The multiplex communication apparatuses (100 and 200), (200 and 300) and (300 and 400) are coupled with each other by means of digital multiplex lines 50, 60 and 70, respectively. The PBX's 1 to 4 are connected to telephone set terminals T1.sub.1 to T1.sub.n, T2.sub.1 to T2.sub.n, T3.sub.1 to T3.sub.n and T4.sub.1 to T4.sub.n, respectively.
Symbols #111 to #411 in the blocks 1 to 4 represent the selection numbers of PBX1 to PBX4 respectively, while the seven digit numbers under the respective telephone terminals represent the selection numbers of the respective telephone terminals.
In such a system, when it is desired to call the telephone terminal T4.sub.1 from the telephone terminal T1.sub.1, the operation is as follows. That is, when selection number information indicative of (411-1111) is sent out from the telephone terminal T1.sub.1, the PBX1 receives the information, detects that the upper three digits (411) of the information do not coincide with the selection number (111) of the PBX1 itself, and sends the selection number information (411-1111) onto the trunk line 10. This information in turn is transmitted to the PBX2 through the multiplex communication apparatus 100, the digital multiplex line 50, the multiplex communication apparatus 200 and the trunk line 20. When PBX2 receives the selection number information (411-1111), it detects the incoincidence of its own selection number (211) with that of the received information and sends out the selection number information (411-1111) onto the trunk line 21. Similarly, PBX3 detects the incoincidence of the information that is finally sent to PBX4 where the coincidence is detected. PBX4, when detecting the coincidence, detects that the telephone terminal T4.sub.1 corresponding to the lower four digits (1111) of the input selection number information is in its idle state and generates a ringing signal for the terminal T4.sub.1.
Meanwhile, the multiplex communication apparatuses 100 to 400 each have an encoder/decoder section for compression transmission of PCM voice codes, that is, a section which decodes a voice signal coming from the associated digital multiplex line and sends it to the related PBX while encoding a voice signal sent from the related PBX and sends it out onto the associated digital multiplex line. For this reason, this system has had such a problem that, when a voice signal is to be transmitted through a plurality of such multiplex communication apparatuses, this requires the repetitive encoding and decoding of the signal, thus deteriorating the quality of the voice signal. For instance, where a voice signal sent from PBX 1 is transmitted to PBX 4, useless encoding and decoding operations of the signals are repeated by the multiplex communication apparatuses 200 and 300, thus deteriorating the quality of the voice signal. Further, when the encoding system employed involves a relatively large time delay in the encoding and decoding operations, the repetition of the encoding and decoding operations disadvantageously results in the fact that the delay time is increased.
In this way, such a prior art multiplex communication system as mentioned above, in which the multiplex communication apparatuses are arranged respectively to decode a voice signal coming from the associated line send it onto the associated PBX while encode a voice signal sent from the PBX and send it onto the line, has been defective in that the exhibition of the relay function of the PBX's requires the repetition of the encoding and decoding operations with the result of deterioration of the voice quality and the increase of the delay time of the voice signal.
Therefore, in view of the above problems in the prior art multiplex communication system, it is an object of the present invention to provide a novel multiplex communication system which eliminates the problems in the conventional system which can prevent the deterioration of the quality of a voice signal even during the PBX relay operation and also can prevent the increase of a delay time of the voice signal.